What is DEF, or Diesel Exhaust Fluid, and why do you need it?


Diesel Exhaust Fluid (DEF) is a non-hazardous solution, which is 32.5% urea and 67.5% de-ionized water. DEF is sprayed into the exhaust stream of diesel vehicles to break down dangerous NOx emissions into harmless nitrogen and water. This system is called Selective Catalytic Reduction (SCR) and can be found on 2010 and later model year trucks and many diesel pickups and SUVs.

DEF is not a fuel additive and never comes in  to contact with diesel. It is stored in a separate tank, typically with a blue filter cap. Small quantities of DEF are injected into the exhaust upstream of a catalyst, where it vaporizes and decomposes to form ammonia and carbon dioxide. The ammonia (NH3) is the desired product which i conjunction to the SCR catalyst, converts the NOx to harmless nitrogen (N2) and water (H20).